Not Applicable.
Not Applicable.
In drilling a borehole in the earth, such as for the recovery of hydrocarbons or for other applications, it is conventional practice to connect a drill bit on the lower end of an assembly of drill pipe sections which are connected end-to-end so as to form a xe2x80x9cdrill string.xe2x80x9d FIG. 1 includes a drilling installation having a drilling rig 10 at the surface 12 of a well, supporting a drill string 14. The drill string includes a bottom hole assembly 26 (commonly referred to as a xe2x80x9cBHAxe2x80x9d) coupled to the lower end of the drill string 14. The BHA includes the drill bit 32, which rotates to drill the borehole. As the drill bit 32 operates, drilling fluid or mud is pumped from a mud pit 34 at the surface into the drill pipe 24 and to the drill bit 32. After flowing through the drill bit 32, the drilling mud rises back to the surface, where it is collected and returned to the mud pit 34 for filtering.
Modern drilling operations demand a great quantity of information relating to the parameters and conditions encountered downhole to permit the driller to change the direction of drilling to find or stay in formations that include sufficient quantities of hydrocarbons. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as xe2x80x9clogging,xe2x80x9d can be performed by several methods.
Logging has been known in the industry for many years as a technique for providing information regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or xe2x80x9csondexe2x80x9d is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed steel cylinder for housing the sensors, which hangs at the end of a long cable or xe2x80x9cwireline.xe2x80x9d The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface, and control signals from the surface to the sonde. In accordance with conventional techniques, various parameters of the earth""s formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
While wireline logging is useful in assimilating information relating to formations downhole, it nonetheless has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drillstring and bottomhole assembly first must be removed or xe2x80x9ctrippedxe2x80x9d from the borehole, resulting in considerable cost and loss of drilling time for the driller (who typically is paying daily fees for the rental of drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, the drilling service company must at times make decisions (such as the direction to drill, etc.) possibly without sufficient information, or else incur the cost of tripping the drillstring to run a logging tool to gather more information relating to conditions downhole. In addition, because wireline logging occurs a relatively long period after the wellbore is drilled, the accuracy of the wireline measurement can be compromised. As one skilled in the art will understand, the wellbore conditions tend to degrade as drilling fluids invade the formation in the vicinity of the wellbore. Consequently, a resistivity tool run one or more days after a borehole section has been drilled may produce measurements that are influenced by the resistivity of the mud that has invaded the formation. In addition, the shape of the borehole may begin to degrade, reducing the accuracy of the measurements. Thus, generally, the sooner the formation conditions can be measured, the more accurate the reading is likely to be. Moreover, in certain wells, such as horizontal wells, running wireline tools can be problematic.
Because of these limitations associated with wireline logging, there is an emphasis on developing tools that can collect data during the drilling process itself. By collecting and processing data and transmitting it to the surface real-time while drilling the well, the driller can more accurately analyze the surrounding formation, and also can make modifications or corrections, as necessary, to optimize drilling performance. With a steerable system the driller may change the direction in which the drill bit is headed. By detecting the adjacent bed boundaries, adjustments can be made to steer the drill bit in an oil bearing layer or region. Moreover, the measurement of formation parameters during drilling, and hopefully before invasion of the formation, increases the usefulness of the measured data. Further, making formation and borehole measurements during drilling can save the additional rig time which otherwise would be required to run a wireline logging tool.
Designs for measuring conditions downhole and the movement and the location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as xe2x80x9cmeasurement-while-drillingxe2x80x9d techniques, or xe2x80x9cMWD.xe2x80x9d Similar techniques, concentrating more on the measurement of formation parameters of the type associated with wireline tools, commonly have been referred to as xe2x80x9clogging while drillingxe2x80x9d techniques, or xe2x80x9cLWD.xe2x80x9d While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used generically with the understanding that the term encompasses systems that collect formation parameter information either alone or in combination with the collection of information relating to the position of the drilling assembly.
The measurement of formation properties during drilling of the well by LWD systems thus improves the timeliness of measurement data and, consequently, increases the efficiency of drilling operations. Typically, LWD measurements are used to provide information regarding the particular formation through which the borehole crosses. Currently, resistivity tools are logging sensors or tools that commonly are used as part of either a wireline or an LWD system. For a formation to contain hydrocarbons, the rock comprising the formation must have certain well known physical characteristics. One characteristic is that the formation has a certain measurable resistivity (the inverse of conductivity). This resistivity can be determined by sending an electromagnetic wave signal of a particular frequency that travels through the formation. As will be apparent to one skilled in the art, a wave traveling from point A to point B through a formation is attenuated and its phase is shifted proportional to the conductivity of the formation. Analysis of this attenuation and phase shift provides the resistivity of the formation surrounding the resistivity tool, which then can be used in combination with other measurements to determine whether the formation will produce hydrocarbons.
Ordinarily, a well is drilled vertically for at least a portion of its final depth. The layers, strata, or xe2x80x9cbedsxe2x80x9d that make up the earth""s crust are generally substantially horizontal. Therefore, during vertical drilling, the well is substantially perpendicular to the geological formations through which it passes. A sudden measured change in resistivity by a resistivity tool generally indicates the presence of a bed boundary between layers. For example, in a so-called xe2x80x9cshaleyxe2x80x9d formation with no hydrocarbons, the shaley formation has a very low resistivity. In contrast, a bed of oil-saturated sandstone is likely to have a much higher resistivity.
FIG. 2 shows a conventional resistivity tool 220 as part of a bottomhole assembly. A well bore 200 is drilled through formation 205, and contains a drill string 210. Attached to drill string 210 is drill bit 215. The resistivity tool includes a transmitting loop antenna Tx that transmits electromagnetic signals into the formation. The resistivity tool also includes a pair of loop antennas, R1 and R2, positioned predetermined distances from the transmitter. Transmitter Tx generates an electromagnetic (EM) wave 255 at a selected frequency that is received at receivers R1 and R2 after traveling through the formation 205. First and second signals at the receivers result. The amplitude ratio and the phase difference of the EM wave can then be measured and a resistivity measurement derived for a particular depth. Thus, the tool indicates the presence of a bed boundary by the rapid change in value of the resistivity measurements.
FIG. 3 shows a depth-resistivity log as measured for a sample wellbore by a conventional resistivity tool, such as shown in FIG. 2. Along the x-axis, resistivity measurements range between 0.2 and 200 ohms. Along the y-axis, a depth reading ranges from about 1010 feet to 1040 feet. Between an xe2x80x9cAxe2x80x9d depth of about 1018 feet and a xe2x80x9cBxe2x80x9d depth of about 1023 feet, the measured resistivity rises substantially, indicating the presence of a bed boundary somewhere between these depths. However, although the resistivity tool that made these measurements indicates a bed boundary somewhere between depth xe2x80x9cAxe2x80x9d and depth xe2x80x9cBxe2x80x9d, the exact depth for the bed boundary is unclear. The best guess for this bed boundary depth is called the xe2x80x9cinflection point.xe2x80x9d The lack of resolution regarding the depth of bed boundaries is particularly troublesome when drilling through a series of relatively thin beds. FIG. 4 is a depth-resistivity graph as measured for a sample wellbore having numerous adjacent thin beds of varying resistivities. Along the x-axis, resistivity measurements range between 0.2 and 200 ohms. Along the y-axis, a depth reading ranges from about 1005 feet to about 1035 feet. Between a depth of about 1018 feet and 1032 feet there are numerous thin beds of varying resistivities. However, because the vertical resolution of the conventional resistivity tool is so poor, it is extremely difficult to establish with any accuracy the exact depth of each bed boundary. Further, even if a bed boundary depth may be determined, it is unclear whether the tool is entering a lower or higher resistivity bed layer.
Another problem with the conventional resistivity tool is an inability to measure resistivities deep into the formation surrounding the borehole. Generally speaking, it is desirable for the resistivity tool to measure at multiple depths into the formation around the borehole between the transmitter and receiver pair. This is referred to as the radial resolution of the tool. Referring to FIG. 5, the first and closest diameter of investigation relative to the resistivity tool is the area within the wellbore through which drilling mud flows back to the surface. If the resistivity of this area is measured inside the wellbore (around the tool itself), a resistivity value will be obtained that generally approximates the resistivity of the drilling mud, Rm. This diameter of investigation can be referred to as Dm, to denote that this is the depth of investigation that will produce a resistivity reading of the drilling mud. The next general area of investigation is the region within the surrounding formation that has been invaded by the drilling mud. This diameter of investigation can be referred to as Di, because a resistivity measurement in this region will produce a resistivity value of approximately Rxo, which is the resistivity of the invaded zone. The third region of investigation for a resistivity tool, is the formation which has not been invaded by drilling mud. A resistivity measurement of this region will yield the true resistivity value of the formation, Rt. As one skilled in the art will understand, the diameters of investigation, Dm and Di will vary depending upon many factors, including the position of the tool in the wellbore, the characteristics of the formation and the drilling mud, the time that has elapsed from when that portion of the wellbore was drilled, and the like. While information regarding Rm and Rxo are useful for purposes of evaluation, one of the goals of the resistivity tool is to measure the true formation resistivity, Rt. Thus, it is important to design the resistivity tool to have a sufficient depths of investigation to measure this resistivity. Ideally, this tool would also measure the resistivity of Rm, Rxo, and Rt at many varying radial depths.
In an attempt to improve radial resistivity data, it is known to add transmitters to the resistivity tool. FIG. 6 includes a resistivity tool 600 including first transmitter T1, at axial location 610, second transmitter T2 at axial location 620, third transmitter T3 at axial location 630, fourth transmitter T4 at axial location 640, first receiver R1 at axial location 650 and second receiver R2 at axial location 660. First, second, third and fourth transmitters are each spaced 8xe2x80x3 from one another. First receiver R1 is spaced 8xe2x80x3 from the first transmitter T1. Second receiver R2 is 8xe2x80x3 from first receiver R1. A measurement point 655 is halfway between first receiver R1 and second receiver R2. Thus, first, second, third, and fourth transmitters are 12, 20, 28, and 36 inches from measurement point 655, respectively. FIG. 7 is a resistivity-invasion depth graph showing data curves that might be obtained with the four transmitter resistivity tool of FIG. 6. These data curves correspond to transmitter-measurement point spacings of 12, 20, 28 and 36 inches. Nevertheless, this amount of data is still does not indicate to the desired degree the resistivities of the formation surrounding the borehole.
It would be desirable to develop a resistivity tool or method that can accurately determine the exact depth of bed boundaries. Ideally, such a tool or method could also indicate whether the resistivity tool is entering a higher or lower resistivity layer, even where numerous thin beds are adjacent to one another. In addition, it is desirable for such a resistivity tool to obtain an increased amount of data with respect to the radial resistivities surrounding the borehole.
A preferred embodiment of the invention features a logging while drilling measurement tool including first transmitter capable of generation of a signal, a first receiver, a second receiver, and a third receiver, the first and second receivers defining a first measurement location corresponding to a first phase shift for the signal, and the second and third receivers defining a second measurement location corresponding to a second phase shift for the signal, and a processor operating on the first and second phase shifts to locate a change in resistivity near the second measurement location. Preferably, this operation includes taking a difference between the first and second phase shifts. This tool can establish whether the tool is entering a relatively high resistivity region or a relatively low resistivity region.
The invention may also be described as a method to determine bed boundaries in a multi-layer formation, including measuring a phase shift of a travelling signal at a first location, measuring a phase shift of that travelling signal at a second location, and operating on the first phase shift and the second phase shift to determine if the first location corresponds to a different resistivity than the second location.
Thus, the embodiments of the invention comprise a combination of features and advantages that enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.